Calibrating a Fluke 8050A?

[slow_rider]

I picked up a 8050A for $12 at a local "junk store". Since the price was low I had figured that something was wrong with it and the place does not allow extensive testing on-site. I took it home and saw that everything seems to be functional, however the measurements are no accurate at all. After cleaning the switches with contact cleaning spray and checking all the voltages on the device it seems that they are OK more or less. It seems some rails are a bit off by 0.5V or so. Not sure how to fix it at the moment, however what I am looking at is the measurments are not accurate and I am wondering if it worth it to calibrate on my own? The manual refers to a DMM calibrator by Fluke which obviously I don't have and I don't think I can provide precision "anything" for calibrations as I don't really trust my other gear. What should I do?

[KerryW]

See http://conradhoffman.com/mini_metro_lab.html for a start.

How do you know it is not accurate now?

You might wany to check the solder connections of the input jacks.

[wiss]

Second best would be to make it agree with your other meters and to be consistent between ranges!

[edavid]

I picked up a 8050A for $12 at a local "junk store". Since the price was low I had figured that something was wrong with it and the place does not allow extensive testing on-site. I took it home and saw that everything seems to be functional, however the measurements are no accurate at all. After cleaning the switches with contact cleaning spray and checking all the voltages on the device it seems that they are OK more or less. It seems some rails are a bit off by 0.5V or so. Not sure how to fix it at the moment, however what I am looking at is the measurments are not accurate and I am wondering if it worth it to calibrate on my own? The manual refers to a DMM calibrator by Fluke which obviously I don't have and I don't think I can provide precision "anything" for calibrations as I don't really trust my other gear. What should I do?

What does "no accurate at all" mean?  If it is off by more than say 1%, that means it's broken.  Don't try to calibrate it until you fix it.

[slow_rider]

Well it seems things just took a turn to the worse... I've noticed some issues with a few of the range buttons so I tried to apply some contact spray I had around for a while and never got around to use it. I applied a generous amount and made sure the spray goes into the switches. I let the whole thing dry for 1 hour and closed up the meter, now I am getting "1" on all parameters on all ranges 
This makes me very sad... so I will open it again and check the voltages 1st and see from there. I could not see a spec. on the voltage rails when I come to think about it...

Update - I connected the meter to power again for taking voltage readings on the rails. I tested the DC voltage and it won't read anything at all on all ranges. On the resistance range I can get 8.825k (not very stable, the LSB keeps on drifting down slowly) but the resistor is (measured) 7.5k so something is not OK.

Test results:
Ohm range / Ohm display
200  -> 1.45
2k    -> .0012
20k  -> 0.001
200k -> 00.00
2000k -> 000.0
20M    -> 0.000

Voltage Range / Voltage input / Voltage display
200m -> 188m -> 84.41m
2         -> 1.984 -> 0.0843
20       -> 19.34 -> +1
200     -> 31.91 -> +1 HV
2000  -> 31.91 -> +1HV

That's what I've tested so far. Does not look good.

Oh, and voltages:

TP2 (13V) - Started at 13.0V when I powered up and than started to get more and more noisy and now drifts from 12.2 to 13.1V. Does not seem very DC to me so I connected my old scope and took a picture. See TP2.jpg The period is about 11 or 12 ms so that's around 83 to 90 Hz.

TP3 (6v) - Around 6.14V but drifts up and down, scope image is the same as TP2.

TP4 (-5V) - Around -5V, same drifts and AC as above.

TP5 (-10V) - Around -11V to -12V.  Same drifts and AC as above.

The AC on the said test points is about 5.1V ppk. Should I swap the power input filtering caps 1st?

[TimNJ]

I'm sorry to hear about your meter. Hope you can fix the problem. Perhaps if it hasn't been turned on in a while, something in the power supply went kaput.

I was intrigued by the link KerryW posted about the Mini-Metrology Lab. I found it kind of hard to read, because they were all separate jpg images. So I compiled the jpgs into a pdf if anyone wants to read it that way.

http://ge.tt/2Jeo5NA2/v/0?c

[Vgkid]

What does the resistance read with no resistor being measured?

[slow_rider]

It took me a while to modify my last post, please see what I've found.

[Vgkid]

Not sure what your scale division is, but there is something wrong with your power supply.
also 80-90HZ, where are you located?

[slow_rider]

The vertical scale is 1 volt per div., the freq. does seem very strange. I'm in Israel and the mains freq. is 50Hz. It might be a little hard to see but the wave shape is not a pure sine.

[Vgkid]

That is not good, start going through the PSU section.

[TimNJ]

Perhaps the main filter cap feeding all of the rails is bad?

[slow_rider]

I've attached a partial schematic of the power supply. You can see the input transformer has some taps that go into a bridge rectifier and into various voltage regulators. I tested the rectifier and all 4 diodes seem to be OK, I measured the AC voltage on the "+" output of the said rectifier and it has about 3.5VAC which I could also spot after the 6V regulator. The same problem was apparent on the "-" output so my 1st guess is that those filter caps are long gone. I'll remove them and test them using my LCR meter.

TimNJ - Didn't see your reply, you've beat me to it!

[KerryW]

I've had issues with solder joints in my 8012A.  You might try reflowing the joints on each of the caps before replacing them.

[wiss]

I've had problems with contact-spray before, the one I used did not specify what was in it and one Fluke 8000 did not work at all after use. Now I only use one which claims to be only (mostly) IPA. Maybe your contact spray contains something leaving a conductive residue?

[Vgkid]

@ Wiss
Since you brought it up, I have had similar issues.
@slow_Rider
after you fix the psu, try cleaning the board/switches in IPA(isoproply alcohol).
If  you have another/ multiple powersupplies: desolder the caps, and try powering those rails seperately.

[slow_rider]

I removed two of the caps that are installed before the regulators and tested them, seems OK to be honest. I am not sure if that is the problem.
My contact cleaner is said to have only 3 ingredients: Propane, isopropanol (rubbing alcohol) and methanol. Don't think any of these should leave residues after it evaporates.

The switches do have some crusty brownish stuff around them, not sure if it is bad or not. It was there when I bought the device. I can take a picture of it. Have no idea how to clean it or test if it is conductive or not and at what freq.

I can post a pic.

[wiss]

Possibly the methanol could be a problem, someone with more chemistry knowledge than me will have to help out here...